in 2015 within the space of just three months we got not just the first one or
the first two but the first three Bell test experiments that claimed to contain
no loopholes. These experiments appeared to show that two particles at different
physical locations can act as a single entity. Surely now the eighty-year-old
debate was over, spooky action at a distance must really be happening or was
it just possible that there was a different a very plausible loophole that
they had all missed? Bell test experiments are not about bells they
take their name from the physicist John Bell who devised a scenario for which
Niels Bohr's quantum mechanics would produce different results to the thought
experiment proposed by Einstein Podolsky and Rosen. John bells idea can be a bit
difficult to get your head around but hopefully the following description will
be relatively easy to follow. First consider natural light passing through a
polarizing filter the filter reduces the intensity of the light by 50% and the
natural assumption is that only half the photons managed to pass through. If we
then add a second filter where there's an angle of 22.5 degrees between the
filter alignments then the intensity reduces by a further 15% and again we
assume that some photons fail to pass through the second filter. From a quantum
mechanics viewpoint each photon travels as a wave of probability and so the
probability of it passing through both filters can be calculated as cosine
squared of the angle between the two filters but from a classical physics
viewpoint each photon of light has its own polarization. After passing through
the first filter all the photons become vertically polarized and then at the
second filter the intensity out is calculated as the
intensity in times the cosine of the angle between the polarization of the
light and the axis of the polarizer. Now we separate the two filters and we send
one of the entangled pair of photons through each of them the entangled
photons are supposed to act as one entity and so if photon A passes through
its filter then by spooky action at a distance photon B will act as though it
has the same polarization as photon A's filter but if photon A fails to pass
through its filter then photon B will act as though it has a polarization
perpendicular to the alignment of photon A's filter. John Bell realized how this
could be used to get results that would be impossible for classical physics to
reproduce. Instead of photons consider a stream of coins being past to three people
who then flip the coins let's call these three people angles the first person is
a particular angle that we'll call angle A the second person is a slightly
different angle that will call angle B the third's another angle that will call
angle C now if all three people flip a coin at the same time there are eight
possible outcomes note that we denote heads as one and tails as zero so zero
zero zero means they all flipped tails and 1 1 1 means they all flipped heads if
the three people keep flipping coins then each result will always be one of
these eight rows next we identify the rows where A flipped heads and B flipped
tails which is denoted by the column heading of A not B then we identify
where B flipped heads and C flipped tails
and finally we identify where a flipped heads and C flipped tails. Now if we
wanted to count the number of times A flipped heads and B flip tails then we
would need to add up the number of times these two results occurred and if we
wanted to add the number of times B flipped heads and C flipped tails then
we add the count for the relevant rows giving us the count for these four rows
to add up but now consider the count for A flipped heads and C flip tails. Since
both the relevant rows are already part of the four rows we've already counted
we know that this count cannot exceed the count for all four rows
mathematically we can say that the sum of the first two counts must be greater
than or equal to the third count. Indeed if all three people flip at the same
time then it's impossible to violate this inequality but what if we set up
some trials where just two people flip a coin at any one time. On one side we
could select between angle A and angle B and on the other side we could select
between angle B and angle C. From all the trials with angle A and angle B we can
get the number where A is heads and B is tails and from the trials with angle B
and angle C we can get the number where B is heads and C is tails and from the
trials with angle A and angle C we can get the number where A is heads and C is
tails Bell realized that if there was no
spooky action going on then this should still not violate the inequality but if
spooky action was going on then it should be possible to select some angles
that would cause this inequality to be violated. If we rewrite this inequality
as probabilities instead of numbers then this forms the original expression known
as Bells inequality.
Now let's sketch out an experiment where we can select between two angles at each
side. We could use a polarizing beam splitter on each side so that a photon
passing straight through can be thought of as heads and a reflected photon can
be thought of as tails. Other forms of Bell equality were devised the most
popular being known as the CHSH inequality. This inequality requires 100%
detection and this cannot be achieved in a photon based experiment. But this
inequality was used by the first of the three loophole free experiments which
was based on the measurement of electron spin. But the second and third loophole
free experiments were based on photons and so they used just one detector at
each side. Then they used an inequality known as the Eberhard inequality this
uses some counts of where only one of an entangled pair of photons is detected
With these photon based experiments, could the observed results be explained
by classical physics? This video was brought to you by Extreme Finitism dot com.